1 A complete example

CL-Yacc exports its symbols from the package yacc:

(use-package '#:yacc)

A parser consumes the output of a lexer, that produces a stream of
terminals. CL-Yacc expects the lexer to be a function of no arguments
(a thunk) that returns two values: the next terminal symbol, and
the value of the symbol, which will be passed to the action associated
with a production. At the end of the input, the lexer should return
nil.

As this grammar is ambiguous, we need to specify the precedence and
associativity of the operators. The operators * and /
will have the highest precedence, + and - will have
a lower one. All operators will be left-associative.

If no semantic action is specified, CL-Yacc provides default actions
which are either #'list or #'identity, depending on how
a production is written. For building a Lisp-like parse tree with
this grammar, we will need two additional actions:

2 Reference

2.1 Running the parser

The main entry point to the parser is parse-with-lexer.

— Function: parse-with-lexer lexer parser

Parse the input provided by the lexer lexer using the parser
parser.

The value of lexer should be a function of no arguments that
returns two values: the terminal symbol corresponding to the next
token (a non-null symbol), and its value (anything that the associated
actions can take as argument). It should return (values nil
nil) when the end of the input is reached.

The value of parser should be a parser structure, as
computed by make-parser and define-parser.

2.2 Macro interface

Generates a grammar and binds it to the special variable name.
This has the side effect of globally proclaiming name special.

Every production is a list of a non-terminal symbol and one or more
right hand sides. Every right hand side is either a symbol, or a list
of symbols optionally followed with an action.

The action should be a non-atomic form that evaluates to a function in
a null lexical environment. If omitted, it defaults to
#'identity in the first form of rhs, and to #'list
in the second form.

The legal options are:

:start-symbol

Defines the starting symbol of the grammar. This is required.

:terminals

Defines the list of terminals of the grammar. This is required.

:precedence

The value of this option should be a list of items of the form
(associativity . terminals), where
associativity is one of :left, :right or
:nonassoc, and terminals is a list of terminal symbols.
Associativity specifies the associativity of the terminals, and
earlier items will give their elements a precedence higher than that of
later ones.

— Macro: define-parser name option... production...

Generates a parser and binds it to the special variable name.
This has the side effect of globally proclaiming name special.

The syntax is the same as that of define-grammar, except that
the following additional options are allowed:

:muffle-conflicts

If nil (the default), a warning is signalled for every
conflict. If the symbol :some, then only a summary of the
number of conflicts is signalled. If T, then no warnings at
all are signalled for conflicts. Otherwise, its value should be a
list of two integers (srrr), in which case a summary
warning will be signalled unless exactly sr shift-reduce and
rr reduce-reduce conflicts were found.

:print-derives-epsilon

If true, print the list of nonterminal symbols that derive the empty
string.

:print-first-terminals

If true, print, for every nonterminal symbol, the list of terminals
that it may start with.

:print-states

If true, print the computed kernels of LR(0) items.

:print-goto-graph

If true, print the computed goto graph.

:print-lookaheads

If true, print the computed kernels of LR(0) items together with their
lookaheads.

2.3 Functional interface

The macros define-parser and define-grammar expand into
calls to defparameter, make-parser, make-grammar
and make-production with suitable make-load-form magic
to ensure that the time consuming parser generation happens at
compile time rather than at load time. The underlying functions are
exported in case you want to design a different syntax for grammars,
or generate grammars automatically.

— Function: make-production symbol derives &key action action-form

Returns a production for non-terminal symbol with
right-hand-side derives (a list of symbols). Action is
the associated action, and should be a function; it defaults to
#'list. Action-form should be a form that evaluates to
action in a null lexical environment; if null (the default), the
production (and hence any grammar or parser that uses it) will not be
fasdumpable.

Computes and returns a parser for grammar grammar.
discard-memos specifies whether temporary data associated with
the grammar should be discarded. Muffle-conflicts,
print-derives-epsilon, print-first-terminals,
print-states, print-goto-graph and print-lookaheads
are as in define-parser.

2.4 Conditions

CL-Yacc may signal warnings at compile time when it finds
conflicts. It may also signal an error at parse time when it finds
that the input is incorrect.

2.4.1 Compile-time conditions

If the grammar given to CL-Yacc is ambiguous, a warning of type
conflict-warning will be signalled for every conflict as it is
found, and a warning of type conflict-summary-warning will be
signalled at the end of parser generation.

— Condition: conflict-warning kind state terminal

Signalled whenever a conflict is found. Kind is one of
:shift-reduce or :reduce-reduce. State (an
integer) and terminal (a symbol) are the state and terminal for
which the conflict arises.

— Condition: conflict-summary-warning shift-reduce reduce-reduce

Signalled at the end of parser generation if there were any conflicts.
Shift-reduce and reduce-reduce are integers that indicate
how many conflicts were found.

— Condition: yacc-compile-warning

A superclass of conflict-warning and conflict-summary-warning,
and a convenient place to hook your own condition types.

2.4.2 Runtime conditions

If the output cannot be parsed, the parser will signal a condition of
type yacc-parse-error. It should be possible to invoke a
restart from a handler for yacc-parse-error in order to trigger
error recovery, but this hasn't been implemented yet.

— Condition: yacc-parse-error terminal value expected-terminals

Signalled whenever the input cannot be parsed. The symbol
terminal is the terminal that couldn't be accepted; value
is its value. Expected-terminals is the list of terminals that
could have been accepted in that state.

— Condition: yacc-runtime-error

A superclass of yacc-parse-error, and a convenient place to
hook your own condition types.

Acknowledgements

I am grateful to Antonio Bucciarelli, Guy Cousineau and Marc Zeitoun
for their help with implementing CL-Yacc.

Copying

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